When you unlock your phone via <**< menu you need to enter a valid unlock code. If you enter wrong codes repeatedly the numbers above will decrement one by one until reach zero(?) and your phone will be blocked:D.
You can unlock for free by a patch, but if you reflash your phone, you need to apply the patch again. Here is a thread about unlocking phones by patch:
http://www.topsony.com/forum/showthread.php?t=4652